Recently, a lot of systems using a band of 800 to 1000 MHz have been employed in mobile radio communications, such as mobile telephone communications and the like. With the advance of such radio communications, coaxial resonators using dielectric ceramics have been availably employed as local oscillators for the communication appliances. Further, with the advance of CATV systems, such resonators have been employed as local oscillators for double super type tuners suitable for multichannel reception.
The aforementioned, prior art local oscillators will be described hereunder with reference to the drawings.
In FIG. 1, the reference numeral 1 designates a part of an AFC detector, in which an AFC output circuit is made up of transistors 2-10 and resistors 11-17 in the form of a current-mirror type. The reference numeral 18 designates an AFC output resistor and the reference numerals 19 and 22 designate resistors for applying a voltage to a variable-capacitance diode 23. The reference numerals 20 and 21 designate resistors for providing an AFC center voltage, the resistor 21 being a variable resistor for adjusting the oscillation frequency. The reference numeral 24 designates a direct-current blocking capacitor, the reference numerals 25 and 26 designate coupling capacitors, the reference numeral 27 designates a resonator using a dielectric, and the reference numeral 28 designates an oscillating circuit.
FIG. 2 is a block diagram of a system related to the oscillator. In FIG. 2, the system comprises a mixer 41, an intermediate-frequency amplifier 42, an AFC circuit 43 and a local oscillator 44. The mixer 41 mixes an input signal with an output signal of the local oscillator 44 so as to generate an intermediate-frequency signal having a frequency which is a difference between the respective frequencies of the input signal and the output signal of the local oscillator 44. The intermediate-frequency signal is amplified by the intermediate-frequency amplifier 42. The intermediate-frequency signal taken out from a part of the intermediate-frequency amplifier 42 is applied to the AFC circuit 43. The AFC circuit 43 detects a deviation of the frequency of the inputted intermediate-difference signal from the original intermediate frequency and applies a detection voltage representing the deviation to the local oscillator 44.
In the following, the operation of the AFC circuit will be described with reference to FIG. 3. In FIG. 3, .circle.B shows a characteristic curve showing the AFC detection characteristic, and .circle.A shows a characteristic line showing the AFC control sensitivity characteristic. If free-running intermediate frequency is shifted to a higher value of f.sub.1 due to a deviation of the oscillation frequency, the frequency is controlled by the AFC function so as to be pulled into a lower frequency value of f.sub.2. Accordingly, the AFC control sensitivity characteristics shown by .circle.A must have the inclination that the intermediate frequency is increased as the control voltage of the AFC circuit increases. Accordingly, in the case of lower-side heterodyne in which the local oscillation frequency is established to be lower than the input signal to the mixer 41 of FIG. 2 by a value of the intermediate frequency, the local oscillation frequency must change in reverse proportion to the variation in the control voltage of the AFC circuit.
Taking the foregoing matters into consideration, the prior art oscillator will be described more in detail with reference to FIG. 1. As described above, the reference numeral 1 designates a part of the AFC detecting circuit in which switching intermediate-frequency carrier signals reversed to each other in phase are respectively applied to terminals A and B and 90.degree. phase-shifted intermediate-frequency carrier signals are respectively applied to terminals C and D so that the AFC detecting circuit is constructed by a double-balanced differential amplifier. The output load circuit is constituted by a current-mirror circuit composed of the transistors 2-10 and the resistors 11-17, so that the DC potential at an AFC detection voltage output terminal E can be set externally. That is, in the current-mirror circuit, the collector current I.sub.1 of the transistor 2 is equal to the collector current I.sub.2 of the transistor 7, and further equal to the collector current I.sub.3 of the transistor 9. On the other hand, the collector current I.sub.4 of the transistor 4 is equal to the collector current I.sub.5 of the transistor 6. Accordingly, if the differential amplifier is well balanced, I.sub.1 becomes equal to I.sub.2, and I.sub.3 becomes equal to I.sub.5, so that even if any external circuit is connected to the terminal E, there is no current delivery between the amplifier and the external circuit. Accordingly, the DC potential of the terminal E, or in other words, the AFC center potential, can be externally desirably established.
The coaxial resonator 27 using a dielectric is connected to the oscillator 28 through the coupling capacitor 26 on one hand, and further connected through the coupling capacitor 25 to a circuit for controlling AFC and for adjusting oscillation frequency on the other hand. An example of the oscillator using such a coaxial resonator 27 is described in "National Technical Report", page 115 through page 123, April, 1985, particularly in detail in FIG. 20a thereof. The AFC center voltage can be suitably established by applying a divisional potential of the resistors 20 and 21 to the AFC voltage output terminal E. At the same time, the oscillation frequency can be suitably adjusted by changing the resistance value of the resistor 21. The output terminal E is connected through the resistor 19 to the anode of the variable-capacitance diode 23. The cathode of the variable-capacitance diode 23 is AC grounded through the capacitor 24, and further connected through the resistor 22 to a source voltage V.sub.CC. The coaxial resonator 27 is considered to be equivalent to a parallel resonator of LC (inductance and capacitance). Accordingly, as the parallel capacitance due to the variable-capacitance diode 23 increases, the oscillation frequency determined by the coaxial resonator 27 and the variable-capacitance diode 23 decreases. In the connection as shown in FIG. 1, therefore, as the AFC voltage increases, the capacitance of the variable-capacitance diode 23 increases, the oscillation frequency decreases, and the intermediate frequency becomes higher as shown by the characteristic line A in FIG. 3.
However, the aforementioned prior art oscillator has a disadvantage in that the oscillation frequency is affected by a variation in the source voltage V.sub.CC. The anode and cathode voltage values V.sub.A and V.sub.K of the variable-capacitance diode 23 are expressed by the following equations: EQU V.sub.A =R.sub.21 .multidot.V.sub.CC /(R.sub.20 +R.sub.21) (3) EQU V.sub.K =V.sub.CC ( 4)
where R.sub.20 and R.sub.21 represent the resistance values of the respective resistors 20 and 21.
The potential difference V.sub.CD between the opposite ends of the variable-capacitance diode 23 is expressed by the following equation: EQU V.sub.CD =V.sub.K -V.sub.A =R.sub.20 .multidot.V.sub.CC /(R.sub.20 +R.sub.21) (5)
The equation (5) depends on the source voltage V.sub.CC, so that the capacitance of the variable-capacitance diode 23 is changed by a variation in the source voltage V.sub.CC. As a result, a problem arises in that the oscillation frequency of the oscillator 28 is affected by a variation in the source voltage V.sub.CC.